Leaked water in Unit 2 of the Fukushima Dai-ichi plant measured 10 million times higher than usual radioactivity levels when the reactor is operating normally, Tokyo Electric Power Co. spokesman Takashi Kurita told reporters in Tokyo.
Radioactivity in the air in Unit 2 measured at 1,000 millisieverts per hour - four times higher than the occupational limit of 250 millisieverts set by the government, he said.
The readings came as workers grappled with how to remove and store the highly radioactive water pooling in four troubled units at the plant.
The discovery of puddles with radiation levels 10,000 times the norm sparked a temporary evacuation of the plant on Thursday. Two workers who stepped into the water were hospitalized with possible burns.
The development set back feverish efforts to start up a crucial cooling system knocked out in a massive March 11 earthquake and tsunami, but has helped experts get closer to determining the source of the dangerous leak.
Chief Cabinet Secretary Yukio Edano, speaking Sunday on TV talk shows, said the radioactive water is "almost certainly" seeping from a reactor core.
Amid a worrying rise in radioactivity at the crippled nuclear plant in Japan, workers are trying out a new tactic to regain control of the situation.
The U.S. is rushing naval barges loaded with fresh water to Japan's overheated Fukushima Dai-ichi plant to try and help workers avoid a full-scale nuclear meltdown.
The switch to freshwater was the latest tactic in efforts to gain control of the six-unit nuclear power plant located 140 miles (220 kilometers) northeast of Tokyo.
Workers at the stricken plant had been using seawater to stabilize reactors, but officials fear salt and other contaminants are clogging pipes and hampering the cooling process.
The U.S. 7th Fleet confirmed that barges loaded with 500,000 gallons of freshwater supplies were dispatched to the Fukushima plant on Saturday.
Experts say the situation at the plant is becoming even more urgent.
Authorities have expanded the evacuation zone from 12 miles to 20 miles after suspicions of a breach surfaced when two workers trying to stabilize the reactor suffered radiation burns.
Plant officials and government regulators say they don't know the source of the radioactive water that caused the burns. It could have come from a leaking reactor core, connecting pipes or a spent fuel pool. Or it may be the result of overfilling the pools with emergency cooling water.
Officials believe Unit 3 is leaking high levels of radiation and workers may have to abandon the plant, and that could mean a full-scale meltdown which would make nearby areas uninhabitable for generations.
"This is huge. For the first time, they're using that dreaded word, breach, meaning uncontrolled release of radiation into the environment," said physicist Dr. Michio Kaku.
Tap water in several areas of Japan, including Tokyo, has shown higher-than-normal levels of radiation. In the capital, readings were at one point two times higher than the government safety limit for infants, who are particularly vulnerable to radioactive iodine.
But levels have fallen steadily since peaking Wednesday, and Tokyo metropolitan officials said Saturday that tap water was now safe for babies to drink.
Just outside a reactor at the coastal nuclear plant, radioactivity in seawater tested some 1,250 times higher than normal, officials said. They also said the area is not a source of seafood and the contamination posed no immediate threat to human health.
However, tests conducted 18 miles (28 kilometers) offshore found radioactive iodine-131 at levels nearing the regulatory limit set by the Japanese government, the International Atomic Energy Agency said. The tests also detected another radioactive substance, cesium-137, at lower levels.
In Los Angeles, the effects of the disaster in Japan are also being felt in a different way. The tourism industry is taking a hit. Japanese tourist are apparently reluctant to leave their troubled country.
"It will be especially noticeable at the hotels that cater to that trade, the places where the tour buses go for example up near Beverly Hills, the beaches and possibly Universal Studios," said Nancy Sidhu with the Los Angeles Economic Development Corp.
The earthquake and tsunami has left over 10,000 dead and more than 17,000 are still missing. Those who have survived the earthquake and tsunami are living without electricity and are quickly running out of food.
The Associated Press contributed to this report.